Goiidruann Reseflrcli, V.2, No. 3 , py. 363-367. 0 1999 Internntionfll Associntiori for Gondiunnn Rcsenrcli, \ n p m ISSN: 7342-937X

GR

Gondwana Research

Exhumation of Granulites Within a Transpressive Regime: an Example from Southern Madagascar J. E. MartelaP, J.M. Lardeaux l, C. Nicollet2 and R. Rak~tondrazafy~

‘ Laborntoire d e Pitrologie et Tecfonique, Universite‘ Claude Bernard, U M R 5570,43 Bd d u 11 Novembre 69622 Villeurbanne, France Dipartenient de Ge‘ologie, Universitt Blake Pascal, U M R 6224,5 Rue Kessler, 63038 Clermont

Ferrand, France ’ Laboratoire dc Pitrologie, Universiti d’Antananarivo, BP 906,101 Madagasacar * Present address: Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Albertov 6, 7 28 43 Prague, Czech Republic ,e-mail [email protected],cz (Mail uscrtpt recciued September 7, 1998; accepted Februauy 24, 1999)

Introduction In Madagascar, a preserved section of continental crust is exposed and offers a n opportunity to study the thermomechanical behaviour of rocks metamorphosed under high-temperature and middle to deep crustal levels (Nicollct, 1990; NedClec et al., 1995;Ashwal et al., 1997).We have established the bulk strain pattern and the P-T-t conditions over a significant area of more than 100 000 km2 in the southern part of Madagascar (Fianarantsoa-IhosyAmpanihy-Fort Dauphin, Fig. 1). Madagascar, a part of Gondwana, represents the eastern front of thc Mozambique belt (Pallister, 1971; Shackleton, 1986). In the southern part of the island, the lithologies consist of postCambrian volcanics and sediments, but highgrade metarnorphic rocks are dominant (Besairie, 1970a and b; Nicollct, 1990). These rocks are represented by orthogneisses, paragneisses, marbles, granitoids, migmatites and iiietabasites metamorphosed under granulite facies conditions. These granulites were highly strained during Pan-African time (580-530 Ma), but do not exhibit any record of the Archean age (Andriamarofahatra et al., 1990; Kroner et al., 1996; Monte1 et al., 1996; Paquette et al., 1994).

Strain Pattern The finite strain pattern of southern Madagascar was derived from thc study of satellite images (20 SPOT scenes) complementcd by structural analysis in the field (foliation, stretching lineation trajectories and kinematics, Martelat et

al., 1995; 1997; Lardeaux et al., 1997). On kilometre scale, the structural pattern is characterized by ductile and fragile structures. Huge faults and caldera in the south-east are well visible. Our studies were focussed on ductile structures. They are defined by intersection of lithological banding and flat topography, and show complex folded structures (domeand-basin) bounded by huge ductile shear zones. Rounded fold geometries more or less elliptic in shape are considered a marker of strain intensity. The spacial evolution of these oval shapes shows axial ratios ranging from 1:1,1:3to 1:20 underlining the regional strain gradients (Martelat et al., 1997). On satellite images, we have studied in detail and precised the limits of six major shear zones (MSZ) 15-25 kilometres wide by more than 100 kilometres long (Ejeda MSZ-2, Ampanihy MSZ-2, Beraketa MSZ-3, BongolavaRanotsara MSZ-4, Zazafotsy MSZ-5, Ifanadiana MSZ-6, Fig. 1)and numerous minor shear zones some 3-5 kilometres wide and less than 100 kilometres long (msz, Fig. 1). This structural pattern results from the superposition of two distinct finite strain patterns D1 and D2. Outside the shear zones, the planar fabric (Sl) is mainly horizontal with dominantly east-west stretching lineation (Ll). Conjugate metric shear zones and F1 folds with horizontal axial planes are compatible with vertical shortening and subordinate westward displacement. Going towards the shear zones the early flat fabric (Sl)becomes folded by kilometric folds (F2) with sub-horizontal hinge line. On a kilometre scale, these open folds progressively evolve to upright and unrooted folds into the shear zones. In the shear zones, the mean foliation is regularly vertical (S2) and the mineral lineation

J.E. MARTELAT ET AL.

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46

44

48"

-22

-24'

-1 MSZ-3

-

-26"

D2

West

D2 -_

Fig. I. Tectonic framework, P-'T conditions and localization of geochronological data in the southern part of Madagascar. This tectonic map is based on satellite imaging and complemented by field studies (Martelat et al., 1997; Pili et al., 1997; Martelat, 1998) and correlated with published geological maps (Besairie, 1970a Pr b). The maximum pressure data are from Nicollet 1988, 1990; Ned6lec et al., 1992 and pers. Communication; Martelat et al., 1997; Martelat, 1998. (1) Major shear zones (MSZ) of -7 Ejeda, -2 Ampanihy, -3 Beraketa, -4 BongolavaRanotsara, -5 Zazafotsy, -6 lfanadiana and minor shear zones (msz), (2) major foliation trends developed in migmatites and rocks metamorphosed under granulite and high ampliibolite facies, (3) post-Cambrian sediments and volcanic rocks, (4) major faults, (5)caldera, (6) numbered circles (1-15) indicate the position o f the geochronological estimates on monazite (Tab. 1). Black squares : towns of Ampanihy A), Betroka B), Fort-Dauphin FD), Fianarantsoa F), Ifanadiana If), Ihosy I), Ranohira R). At the bottom a 3D schematic diagram of the structures shows the interference between D1 and D2 finite strain patterns.

Gondrunnn Iiltinelltal crust defOrnlatiOn leSLllLed from the Oblique convergence of crustal lieterogenrities: the Tanzanian craton Lo the west a n d t h e Dharwar craton to the east, modified after Norton and Sclater, 1979; Drury and Holt, 1980; Powell et al. 1980;Daly, Schakleton, 3986; Kriegsman, 1993; Windley et a!. 1994;Sacks et al. 1997; M,irtelcit, 1998.

Pure Shear

Simple Shear

the convergence of the Tanzanian craton in the west and the Dharwar craton in the east (Fig. 2, Drury and Holt, '1980; Daly, 1986; Scliakleton, 1986; Kriegsman, 1993; Windley et al., 1994; Sacks et al., 1997; Martelat, 1998). In this context, the superposition of the D1 and D2 strain patterns, in the range of 590-500 Ma, is clearly the result of an oblique convergence of cratons promoting the bulk shortening of the intracratonic lithospliere. Under very high-temperature metamorphism (max T > 85OoC),we observe a strong strain partitioning on a regional scale and the D2 strain gradient increasing from the east to the west. Madagascar granulites were mainly exhumed (at least 20 kilometres of uplift) during a transpressive regime (D2 strain pattern). The vertical displacement was controlled by the D2 strain gradient, and was the largest in domains where the ratio of pure shear versus simple shear was the highest (Fig. 3 ) as ~ predicted by tliermonieclianical modelling (Thompson et al,, 1997). This llypotllesis is by tlie regional distribution of litliologies,more granitic in the east and more mafic to the west. The basenlent Of Madagascar/ thus represents an example of deep seated rocks exhumed during an obliquely convergent (transpressive) orogen.

@ T = 750°C

*--

% Mafic Rocks

Fig. 3. Interprelative block diagram showing the D2 dynamic event in the southern p r t o f Madagascar. The two verlical planes in dark grey represent vertical major shear zones of Ampanihy and Beraketa whereas the oblique shear zone corresyoiids t o the Bongolava-Ranotsara structure>. The zone where the exhumation of the lower crust is stronger is located to the west where the strdin regime is pure shear dominated. The maximum pressure is given at a constant temperature of 800OC. Mafic li thologies increase from the east to the west.

Gond7oann Rescwcli, V. 2,Nu. 3, 1999

EXHUMATION OF GRANULITES IN SOUTHERN MADAGASCAR

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